Allowing Bestlin Group to join a fresh round of bidding for 489 city buses would be an affront to the government's anti-graft campaign, warns former finance minister Korn Chatikavanij.
He has urged the prime minister to deal swiftly with any irregularities in the bus procurement.
Writing on his Fcebook page, Mr Korn said he supported the Office of the Auditor-General (OAG)'s caution that Bestlin being permitted to join the bid would violate good governance standards in the public sector.
"If the government makes a concession whereby a private firm accused of irregularities and of taking advantage of the state is still able to enter a deal with the state, I think the government should stop asserting that it is serious about combating corruption," Mr Korn wrote.
The OAG voiced its caution after the transport authority removed Chinese-owned Bestlin from its blacklist despite a tax-avoidance scandal involving an affiliate of the bus supplier. This step clears the way for the company to take part in a new round of bidding scheduled for Thursday.
Super Zara Co, which handles imports for Bestlin, was accused by the Customs Department of making the buses appear as though they were manufactured in and imported from Malaysia, to exploit a tax-exemption benefit under the Asean free-trade agreement.
If the vehicles were made in Malaysia as claimed, Bestlin would be eligible for a 40% import duty exemption worth 1.2 million baht per bus. The Customs Department suspected the vehicles were in fact produced and assembled in China.
After Bestlin failed to deliver the contractually-agreed number of buses on schedule as stipulated in the 3.3-billion-baht contract, the Bangkok Mass Transit Authority (BMTA) scrapped the contract and subsequently put the company on its blacklist.
Like Mr Korn, Auditor-General Pisit Leelavachiropas also said allowing Bestlin to renew its bid to supply the buses would run counter to good governance practices. Mr Pisit said his office was looking into the issue.
Mr Korn wrote Sunday the BMTA appeared "indifferent" to the controversy in the beginning and it was now giving Bestlin another go at the bidding session.
"What does that mean? Can this conceivably be regarded as a serious effort to fight corruption?" Mr Korn asked.
The former finance minister urged Prime Minister Prayut Chan-o-cha to explain whether he thought Bestlin should re-enter the bid.
If the premier decides the firm should not, he should order swift action against those behind the purchase delay scandal.
Last week, Somsak Hommuang, deputy permanent secretary for transport, said the BMTA's legal team was waiting for Bestlin to clarify its procedures to see why it did not complete the contract.
He said that BMTA deputy director Yuk Charupum, serving as the acting director of the BMTA, had taken Bestlin off the blacklist.
On Aug 14, BMTA deputy director Prayoon Chuaykaew said the bidding process was open to Bestlin as it was no longer among firms blacklisted for abandoning work with the government.